RAILWAY TECHNOLOGY FAQ
Welcome to Cecube Frequently Asked Questions. These pages address commonly asked traction related questions. They have a technical bias to varying degrees, dealing with electrical and mechanical matters related to traction drive vehicles such as WSP and minimum pulse width, as well as software and simulation. More general Railway FAQ regarding services, passenger issues and rolling stock class information are covered on numerous other sites. However, if your question is traction specific and is not addressed here please contact us.

TRACTION DRIVES - LIST of FAQ page 2

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Q9. How are slip or creep control systems different to wheel slide protection (WSP) systems?

WSP is an anti-skid function, similar to that found on automobiles. This is in contrast to slip or creep control systems that are analogous to traction control systems commonly found in high performance cars. When the rail surface is contaminated, applying brakes may cause a lock-up, and with very little adhesion the train slides. The purpose of WSP is to reduce the extension in train stopping distances that results. Like anti-skid systems in other vehicle types, WSP systems achieve this by detecting the onset of wheel sliding and releasing the mechanical brakes to unlock the wheels. WSP is invariably effective in reducing stopping distance extension, although the degree of effectiveness is track condition dependent. The WSP function is normally computerized in modern rolling stock (in older stock it could be analogue electronics or a mechanical system) as it too depends on measurement and calculation of wheel speeds. It then controls the pneumatic brakes according to a WSP algorithm.

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Q10. What is the constraint on a switching converter's minimum pulse width and why is it significant?

The shortest allowable time between switching a converter device on and off again and visa versa is referred to as the minimum pulse width. It determines the minimum output voltage resolution (distortion) from one phase of a pulse converter. It is necessary to ensure complete snubber discharge when a device switches on, and to guarantee its successful commutation when switched off. For a GTO (Gate Turn Off thyristor) to change from conduction to blocking requires significant gate energy and time to recover. The GTO device characteristics, as well as the operational voltage and current are major factors setting the minimum pulse width.

Conversely, IGBTs (Insulated Gate Bipolar Transistors) are linear devices operated into saturation, and their switching behavior is faster and unlike a GTO. The gate drive energy required is lower for the IGBT, however, in saturated conduction mode a single device has a higher conduction loss than the equivalent rated GTO. This is overcome by connecting a set of transistors in parallel per device for high power applications, with consequential design issues concerning equal current sharing. The faster IGBT will have a smaller minimum pulse width constraint than the GTO, which permits operation at higher switching frequency without increased voltage distortion.

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Q11. Why do safety failure rate calculations invariably give a predicted result that meets the target figure by only a small margin?

The application of reliability methods continuously refines failure frequency calculations, making them progressively less pessimistic. For example, when performing a credible fault mode analysis the inherent protection is normally evaluated to a point of sufficient reliability, rather than exhaustively considering every detailed protection aspect. It is therefore not coincidental when results comply by a modest margin against the specified target failure rate. This just reflects the effort required to achieve compatibility by preventing unnecessary pessimism.

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Q12. What are conductor and running rail internal and external inductances?

In general any conductor has components of Internal and External Self Inductance that lead to a resultant magnetic field when current is passed through it. Self inductance of a conductor is due to its physical construction and properties and results from internal flux linkage. It does not depend on the proximity of other conductors or the current in them, but for rails self inductance is strongly frequency dependent.

When current passes through a conductor there is also external flux linkage, This component is a function of the conductor dimensions (e.g. radius) and is referred to as External Self Inductance. A further component that may be present is External Mutual Inductance resulting from flux linkage due to coupling between two or more current carrying conductors. This is dependent on the distance between conductors in their respective loops.

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Q13. How are the per phase equivalent circuit parameters for an induction motor determined for a delta connected motor?

The equivalent circuit parameters assume a star connected induction motor by convention. If the motor is delta connected then the AC voltage applied to each phase is √3 greater than the star equivalent. For the power to be the same, the delta phase current must be √3 smaller than the star equivalent.

Hence, Zstar = [1/√3].Vdelta / [√3].Idelta = Zdelta / 3

Therefore, the relationship between actual delta parameter values to equivalent star (or per phase) parameters is Zdelta = 3.Zstar.

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Q14. What are the causes of flash-over in DC traction motors?

Flash-over is a form of electrical discharge caused by insulation breakdown around the commutator of a DC motor or generator.

DC motors use commutators to reverse the current in a winding as it passes under the brushes. The reversal of current is essential to maintain the output torque, in the presence of a DC magnetic flux produced by excitation of the field winding. Typical DC traction motors have in excess of 200 commutator segments (depends on motor voltage rating) to reduce the voltage differential across neighbouring segments and improve the continuity of the motor torque. However, there are scenarios where the current is not reversed in the normal way, which accentuate with speed and results in sparking under the brush as it transfers to an adjacent segment. Apart from poor brush and commutator maintenance, this condition invariably arises from extreme electrical or mechanical disturbance. This includes:

  • Sudden loss of adhesion
  • Momentary supply interruption
  • High voltage energetic transients on the supply
  • Extreme physical impact shocks caused by badly maintained track
  • Excessive motor vibration due to worn mechanical components

Trains travelling at high speed under weak field operation, without limiting resistance or control of armature voltage, are particularly vulnerable. A sudden loss of voltage will reduce the field flux to residual levels. In the case of separately excited (SEP-EX) control, the traction equipment should protect the motor by ensuring the flux is re-established before the re-application of armature voltage. Unfortunately short transient events can defeat this protection and allow the armature to be connected across the supply before the field is re-established. The maximum permissible armature to field current ratio for safe operation is easily exceeded in this scenario.

Series DC motors fare a little better. The armature to field current ratio is fixed by the circuit configuration, according to field divert settings. This makes it less susceptible to current ratio problems in weak field operation, but increases the risk of very rapid wheel and armature acceleration on adhesion loss. Very high armature acceleration simultaneously stresses the motor both electrically and mechanically. Historically, the DC motor has been the lifeblood of electrified railways. However, the cost of commutator manufacture and maintenance to prevent flash-over is a major factor in the trend towards AC motor drives.

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Q15. What is load flow simulation of railways and how does it differ from time domain simulation of electronic circuits, such as those performed by PSpice?

In load flow model analysis trains are considered as loads with power consumption determined from demand and location. Analysis requires a complex form of AC/DC load flow because the behaviour of the AC feed is a vital element affecting a DC railway, or the DC link of a train on an AC railway. This type of simulation is used for both power system design/debugging and for operators to test timetable schedules. The timetable determines the position and load condition of each train at any instant. In practice, timetable uncertainties or system faults may cause unpredicted over-current conditions in either AC or DC lines that create power surges. Prediction of such events is vital to the safe operation of the electrical power distribution system.

AC/DC load flow is a numerical technique, often referred to as Gauss-Seidel (a solution method for linear matrix equations), that determines the steady state solution of such networks. This avoids instantaneous calculation of AC voltage and current waveforms and the transient phase relationship between them. In contrast time domain simulation solutions (such as from a SPICE simulator) will include transient responses computed by a numerical integration engine. These solutions will in general be more accurate than load flow results, but demand greater computational resources to achieve this.

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Q16. Text books indicate mechanical and electrical systems have strong mathematical similarities, especially for second order linear systems, but how are the electrical parameters of inductance, capacitance and resistance analogous to mass, stiffness and damping in a simple mass-spring-damper configuration?

A mass-spring-damper general solution is described by a second order linear equation in displacement x as

x'' + (D/M).x' + K/M = 0

where M = mass, K = spring stiffness and D = damping rate (either due inherent spring damping or that resulting from use of a physical damper.

The equivalent electrical system is the series LCR circuit solved for circuit charge q (where i(t) = dq/dt). This gives the general second order solution of the form

q'' + (R/L).q' + 1/(LC) = 0

Comparing the two solutions shows directly that M is equivalent to L and similarly D to R. The stiffness K is equivalent to 1/C. Therefore, capacitance is analogous to elasticity, being the reciprocal of stiffness.

This is apparently quite straightforward and a helpful way for mechanical engineers to work with simple electrical systems and visa versa. However, this can also be a source of confusion for students. At an elementary level the similarity in diagrammatic representation between a spring and inductance, and a damper with capacitance must be ignored. Secondly, the LCR circuit in question is the series tuned circuit. However, the parallel LCR circuit occurs rather more commonly in electrical circuits such as radio receivers. The parallel circuit is not a simple dual of the simple mass-spring-damper, but this is not necessarily intuitive to the student of mechanical engineering.

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